The present invention relates to frequency trimming of surface acoustic wave (SAW) devices.
SAW devices utilize the localized propagation of acoustic waves on the working surface of a planar piezoelectric substrate. SAW transduction between electrical signals and acoustic waves is accomplished by thin film metallic interdigital electrodes on the substrate surface. A typical SAW device includes a substrate die with associated acoustic channel and transducers on the working surface, adhered to a base and sealingly enveloped within a package including a metal cover.
The SAW device to be delivered to the end user must meet very tight tolerances on the target output frequency. Generally, some form of frequency trimming is employed as a last step during manufacture, such as by fabricating the device for a higher frequency output than the target frequency and then trimming by adding effective mass to the working surface, especially the acoustic channel. SAW device frequency can be trimmed by several methods before the cover is sealed, but once sealed the device is subject to further random frequency shifts due to the sealing process, environmental stress, screening, and aging. These shifts can significantly reduce product yield. In one known but atypical technique for post-seal trimming, material adhered to the inside of a sealed, transparent cover is ablated by a laser onto the acoustic channel, but this is difficult and expensive to implement.